1. Field of the Invention
This invention relates to a process and apparatus for thermoforming hollow articles, such as cups, dishes or lids, and more particularly to an improved process and apparatus for thermoforming hollow articles including concomitant die cutting of the thus formed hollow article.
2. Description of the Prior Art
This invention is particularly concerned with thermoforming operations in which a web or sheet of thermoplastic material is first softened by heat and then formed or shaped while in this stretchable plastic state by being drawn or pressed against the contours of a mold or die under the influence of a fluid pressure differential. The former may be exemplified by the application of pressure within a pressure box or female mold assembly produced by compressed air or the like, to press the softened material against the countours of a male mold or core pin. Alternately, a vacuum may be applied from within a female mold while allowing atmospheric pressure to shape the heat softened thermoplastic material against the surface of the female mold. For many purposes, a combination of these two techniques is preferred with the softened sheet material being subjected to vacuum on the face contacting the mold surfaces and to a substantial positive pressure on its other face as such combination provides greater versatility and better control in the forming operations.
Apparatus for effecting thermoforming operations, in general, is comprised of a press having upper and lower platens on which are mounted a male mold assembly and female mold or pressure box assembly. In this regard, the male mold is generally mounted on the lower platen, however, this is normally a matter of choice. Prior to forming of the sheet of thermoplastic material as hereinabove discussed, a clamping assembly normally constituting a part of the male mold assembly is first caused to trap the sheet of thermoplastic material about the entrance into the female mold or the pressure box assembly including a cooperating clamping grid. Thereafter, the male mold of the male mold assembly is caused to pass through the plane of the thermoplastic material and eventually form the hollow article, as hereinabove discussed. Relative movement of the male and female mold assemblies is effected by hydraulic cylinder or electric motor assemblies together with associated cams, gears, support arms, etc., to move the platens whereas movement of a clamping assembly is effected by an associated cylinder assembly.
The hydraulic cylinder or electric motor assemblies are large even for small thermoforming apparatus and have large energy requirements. In this application the term fluid cylinder assembly is to be interpreted as the combination of a cylinder, piston and rod which is operated by a pressurized fluid, i.e. a gas or a liquid.
Die cutting of the hollow article is generally effected in another apparatus including die cutting elements into which is introduced the web or sheet of thermplastic material including thus formed hollow articles. Die cutting in a separate operation suffers from alignment problems as well as problems associated with die cutting of cooled thermoplastic materials, e.g. splitting and the like, as well as cost considerations, i.e. a separate assembly.
Other known assemblies include the thermoforming and die cutting of the hollow article without indexing of the heated sheet of thermoplastic material, with the article being laterally drawn off and moved upwardly to a stacking device.
In many assemblies, the hollow articles are subjected to further finishing operations in a corresponding appropriate finishing device, e.g. punching, embossing, printing, testing or the like. Such finishing devices are expensive, require space, are time consuming and result in the manufacture of a certain number of unacceptable products.